1. Field of the Invention
The present invention relates to a method and system for the prevention of corrosion of conductive structures using semiconductor technology, particularly where those conductive structures are part of a biomedical device in or on the body.
2. Discussion of the Background Art
A variety of methods for controlling corrosion have evolved over the past several centuries, with particular emphasis on methods to extend the life of metallic structures in corrosive environments. These methods typically include protective coatings which are used principally to upgrade the corrosion resistance of ferrous metals, such as steel, and some nonferrous metals, such as aluminum, and to avoid the necessity for using more costly alloys. Thus, they both improve performance and reduce costs. However, such protective coatings typically have several pitfalls, including poor applicability to non-metallic structures that suffer from corrosion or fouling.
Protective coatings fall into two main categories. The largest of these categories is the topical coating such as a paint, that acts as a physical barrier against the environment. The second category consists of sacrificial coatings, such as zinc or cadmium, that are designed to preferentially corrode in order to save the base metal from attack.
Cathodic protection and coatings are both engineering disciplines with a primary purpose of mitigating and preventing corrosion. Each process is different: cathodic protection prevents corrosion by introducing an electrical current from external sources to counteract the normal electrical chemical corrosion reactions whereas coatings form a barrier to prevent the flow of corrosion current or electrons between the naturally occurring anodes and cathodes or within galvanic couples. Each of these processes provided limited success. Coatings by far represent the most wide- spread method of general corrosion prevention (see Leon et al U.S. Pat. No. 3,562,124 and Hayashi et al U.S. Pat. No. 4,219,358). Cathodic protection, however, has been used to protect hundreds of thousands of miles of pipe and acres of steel surfaces subject to buried or immersion conditions.
The technique of cathodic protection is used to reduce the corrosion of the metal surface by providing it with enough cathodic current to make its anodic dissolution rate become negligible (for examples, see Pryor, U.S. Pat. No. 3,574,801; Wasson U.S. Pat. No. 3,864,234; Maes U.S. Pat. No. 4,381,981; Wilson et al U.S. Pat. No. 4,836,768; Webster U.S. Pat. No. 4,863,578; and Stewart et al U.S. Pat. No. 4,957,612). The cathodic protection concept operates by extinguishing the potential difference between the local anodic and cathodic surfaces through the application of sufficient current to polarize the cathodes to the potential of the anodes. In other words, the effect of applying cathodic currents is to reduce the area that continues to act as an anode, rather than reduce the rate of corrosion of such remaining anodes. Complete protection is achieved when all of the anodes have been extinguished. From an electrochemical standpoint, this indicates that sufficient electrons have been supplied to the metal to be protected, so that any tendency for the metal to ionize or go into solution has been neutralized.
Recent work in the study of corrosion has found that electrochemical corrosion processes appear to be associated with random fluctuations in the electrical properties of electrochemical systems, such as cell current and electrode potential. These random fluctuations are known in the art as xe2x80x9cnoisexe2x80x9d. Researchers have begun to apply noise analysis techniques to study the processes of corrosion in electrochemical systems.
Riffe, U.S. Pat. No. 5,352,342 and Riffe. U.S. Pat. No. 5,009,757 disclose a zinc/zinc oxide based silicate coating that is used in combination with electronics in a corrosion prevention system. The zinc/zinc oxide particles in the coating are disclosed as having semiconductor properties, primarily a p-n junction at the Znxe2x80x94ZnO phase boundary. When reverse biased, this p-n junction is described as behaving as a diode and inhibiting electron transfer across the boundary. This restriction limits electron transfer from sites of Zn oxidation to the sites of oxygen reduction on the ZnO surface. Effectively, there is increased resistance between the anode and cathode of local corrosion cells and corrosion is reduced.
On average, the Znxe2x80x94ZnO based junction will be reversely biased due to the potentials associated with the oxidation of Zn at the Zn surface and the reduction of O2 at the ZnO surface. However, significant stochastic voltage fluctuations occur. These voltage fluctuations cause the junction to episodically become forward biased. When forward biased, electron transfer across the junction increases and there is an acceleration of the oxidation of Zn and reduction of O2. Effectively, there is a short circuit between the anode and cathode of local corrosion cells and corrosion is enhanced.
The Riffe patents disclose attachment of a fixed value capacitor in the electrochemical circuit of the corrosion prevention system. However, there is no way to control the level of capacitance nor any method suggested for determining the level of capacitance needed to effectively prevent corrosion in any given structure. Hence, it is necessary to use an overcapacitance in the system to be effective.
The inside of a living organism""s body is frequently thought of as reflecting the milieu of the sea, where life is thought to have arisen. As noted above, fouling and corrosion of man-made objects in the sea are subjected to a number of deleterious processes including fouling and corrosion. The fouling process is characterized by adherence and growth of both micro-and macro-organisms. Many of these same deleterious processes occur on devices that are either implanted within the bodies of humans and other living organisms or intimately associated with such devices(such as plasmapheresis systems, dialysis units and the like). The use of semiconductor materials in biological settings is only recently starting to bear useful results (see for example, Mirkin et al, Nature, 405, 626 (2000)). Fouling of biomedical devices and surfaces often is in the form of films of bacteria, also known as bacterial biofilms. This phenomenon has been reported by Mittelman, Methods in Enzymology, 310, 534-551 (1999), where the recovery and characterization of biofilm bacteria are described in connection with medical devices. One approach to treatment and control of such bacterial biofilms has been proposed by McLeod et al, Methods in Enzymology, 310, 656-670 (1999), by the use of a combination of electromagnetic fields with antibiotics. Unfortunately, the application of such electromagnetic fields, requiring the establishment of a current that is carefully controlled, is fine for laboratory work, but would be impractical in situ, where the real longterm effects of biofouling would be most felt.
Accordingly, one object of the present invention is to provide a semiconductive and pliable coating that provides anticorrosion and antifouling/antirejection and antibiotic properties to any conductive or nonconductive structure that is placed in the body or associated with biomedical devices.
A further object of the present invention is to provide a method for protecting conductive metallic structures from corrosion that is fine-tuned to the unique characteristics of the metallic or nonmetallic structure and its placement in the body of a living organism.
A further object of the present invention is to provide a method for preventing fouling, infection and corrosion of conductive structures by using semiconductor technology and with no external anode, no electrolyte, and no current flow.
A further object of the present invention is to provide a system for protecting conductive structures from fouling, infection and corrosion within the body, wherein the system provides long term protection with minimal system maintenance required.
These and other objects have been satisfied by the discovery of a semiconductive biologically xe2x80x9cacceptablexe2x80x9d coatings and associated electronic system, wherein the system can be operated by merely filtering voltage fluctuations in the conductive structure on which the semiconductive coating is placed, wherein the method for using the system comprises:
coating the conductive structure with a semiconductive coating with a fixed electronic filter connected to said coated structure,
monitoring noise generated by said coating having said fixed electronic filter connected thereto,
using an adjustable filter connected to said coating to determine an anti-corrosive filter response needed to minimize the noise generated by said coating; and
replacing said adjustable filter with a passive or active filter having a filter response of at least said anti-corrosive filter response,
wherein the conductive structure is a biomedical device attached to or implanted in a subject""s body.